L-carnitine, called also vitamin B.gamma., is a substance necessary for the transportation of fatty acids through mitochondrial membranes in animals. In the cells of animals, there are also acylated derivatives of L-carnitine, namely, acyl-L-carnitines. Acyl-L-carnitines are excreted from the animal and are found in urine. Therefore, quantitative determination of the content of acyl-L-carnitines in blood etc. is very important in, for example, monitoring the existence of functional disorders of mitochondria in order to detect any defect in the intramuscular energy transference in animals.